1. Field of the Invention
This invention relates to containers having an easy opening end (hereafter referred to as EOE) which are used as cans for food or for beverage or other fluids.
2. Description of the Prior Art
Heretofore there have been used two kinds of EOE for metal containers, one kind being made of steel and the other of aluminium. In both EOEs a top wall and a tab are fixed by a mechanically caulked structure. The following is a brief description of the prior art method of constructing an EOE.
A top wall the inside surface of which has been coated in advance is scored on its outside surface in a form to be torn off from the top wall. At the center of the top wall is formed a projection having a diameter of 2 to 5 millimeters and a height of 1 to 5 millimeters.
A tab is formed having two holes punched therein. One hole is to enable the forefinger to remove the removable portion of the top wall defined by the scoring. The other hole is for accepting the projection at the center of the top wall.
After the tab is positioned on the projection of the top wall, the projection is crushed, i.e. caulked, fixing the tab on the top wall.
After the fixing of the tab, the inside surface of the top wall, especially around the caulked projection is recoated.
The top wall of such an EOE made of steel or aluminium or paper sheet is utilized for containers which have a side wall made of steel sheet, aluminium sheet, or plastic sheet. Metal containers made of steel or aluminium sheet are generally divided into 2-piece or 3-piece cans according to the purpose, such as what the can is to contain, the material of the container, and the form of the container.
In case of a 2-piece can, a bottom wall and a side wall are formed in one piece. (Hereinafter such a one-piece side and bottom wall is referred to as a "side body".) Such containers are called DI cans (drawn and ironed can), Drawn cans or DRD cans (drawn and redrawn can).
In case of a 3-piece can, the top, bottom and side walls are formed separately and united.
The side wall of a 3-piece can must be bonded along its axis by soldering, adhesively bonding or welding. For 3-piece cans steel sheet is generally used, because aluminium has poor bonding and welding properties.
With regard to steel containers, the top wall with the EOE is made of steel or aluminium for reasons stated hereinbelow, although the side body of 2-piece cans or the side and bottom walls of 3-piece cans are made of various surface treated steel sheets. Because of this fact, the recent social demand for the recycling of resources cannot be satisfied sufficiently.
The application of the conventional EOE made of steel is limited because of its poor easy opening properties. The reason for the limited application is that the properties needed for the top wall, that little strength is needed to tear off the scored part, is at odds with another property needed for the top wall, that it must have the deep drawability required to affix the tab by caulking.
From this point of view aluminium is suitable for the top wall of EOEs because of its low yield strength and high deep-drawability. But it is very difficult for steel sheet to satisfy the said two properties and therefore it has been said that the easy opening properties are inferior to those of aluminium.
In the conventional EOE the recoating is carried out on the inside surface of the top wall to prevent corrosion caused by the contents of the can. But because of the complex structure of the caulked projection, it is difficult to guarantee a perfect coating on the caulked projection, and occasionally corrosion may appear from there.
One proposal is made by the U.S. Pat. No. 3,946,896, which discloses an EOE having a closure in the form of a filmlike tab with a bottom portion which is welded in a closed loop adhesive bond or weld line about an opening. This type of EOE is quite different from that of the present invention. Furthermore the material of the top wall to be used in the prior art is plastic or alumimium and the weld disclosed in the patent is molecular bonding as disclosed by the claim.